Every single programming language serves one purpose: explain to the computer what we want it to do. HTML is... not a programming language, it's a markup language, which basically means text formatting. XML and JSON are in the same category The rest of languages fall in 4 general categories:
- Assembler is (edit: for every intent and purpose) the native language of the machine. Each CPU has it's own version, and they are somewhat interoperable (forward compatibility mostly).
- system languages(C and C++ ) . They are used when you need to tell the computer what to do, as well as HOW to do it. A program called a compiler interprets the code and transforms it into assembler.
- application languages( Java and C#). Their role is to provide a platform on which to build applications using various standardized ways of working.
- scripting languages(Python, and Perl). The idea behind them is that you can build something useful in the minimal amount of code possible.
Then you have various hybrid languages that fit in between these main categories. The list goes on and on. Various languages are better suited for various tasks, but it's a matter of opinion. Finally and most importantly: Javascript is an abomination unto god, but it's the only language that can be reliably expected to be present in web browsers, so it's the only real way to code dynamic behavior on webpages.